1. Field of the Invention
This invention is concerned with manufacturing a high-capacity servo-valve body with the aid of wire EDM technology.
2. Discussion of the Prior Art
As is well known, servo valves are used to control devices such as seismic vibrators, linear actuators or reversible motors. Typically such valves consist of a valve body having a series of internal radially-arranged, annular flow chambers. The flow chambers are linearly disposed at selected intervals along the length of the valve body. The central flow chamber is fluidly coupled to an inlet port that may be connected to a source of pressurized fluid. The two outermost flow chambers are connected to first and second drain ports that feed into a low-pressure sump. The other two intermediate flow chambers are connected to first and second outlet ports for directing pressurized fluid to the device to be controlled. A bore, of lesser diameter than the diameter of the annular flow chambers, is machined internally along the length of the valve body, orthogonally intersecting the respective annular flow chambers. A valve spool, consisting of three or more spool-shaped lands separated by stem portions, is slidably mounted within the bore. End caps mounted at each end of the bore include drive means for reciprocating the spool valve laterally within the bore. The drive means may be electrically, hydraulically or pneumatically programmed by a pilot valve of any desired type. A linear variable displacement transducer (LVDT) monitors the position of the valve spool within the bore. Depending upon the position of the valve spool, the lands are designed to block off selected flow chambers. A typical servo valve is described in U.S. Pat. No. 4,593,719.
In operation, when the valve spool is centered, the ports are closed and no action occurs. When the valve spool is moved towards one end of the bore, the inlet port is fluidly coupled to the first fluid outlet port while the second outlet port is open to the corresponding drain port so that the controlled device will perform a function in a first direction. With the valve spool at the opposite end of the bore, the inlet port is open to the second outlet port and the first outlet port is opened to the first drain port so that the controlled device performs its function in a second direction.
In one method of construction, the servo valve body is of cast iron or steel. Fluid passageways in the casting must allow for a smooth transition in flow path between the substantially rectangular annular flow chambers and the circular inlet and outlet pipe fittings demanded by conventional plumbing components. The casting is internally complex and it is difficult to machine the septa between the flow chambers to provide fluid ports with the tolerances required by the lands on the valve spool. Furthermore, cast metals are too soft to retain the sharp shoulders on the septa that are needed for accurate fluid-flow control. Accordingly, the bore is usually honed to a desired diameter. A cylindrical sleeve of hard tool steel is provided, having a plurality of radial ports machined in its wall to communicate with the respective fluid flow chambers. The radial ports in the sleeve are dimensioned to match the lands on the valve spool. The sleeve is inserted through the bore, sealed thereto with gaskets or O-rings. The valve spool then reciprocates inside the sleeve. Alternatively, the sleeve may be inserted and sealed in place using heat-shrink technology. The assembly tends to be bulky because the fanout from the flow chambers to the circular plumbing connections takes up considerable space.
For small servo valves of relatively low capacity that are manufactured in quantity, the above method of manufacture is economical. For large valves that need to be custom-made in only one or two copies, creation of the required molds for casting the valve body is extremely complex and expensive. This invention provides a method for economically making a precision servo valve body that is compact, simple of construction and assembly and that does not requires special castings or sleeves.